Little's law can be used to describe a system in steady state from a queuing perspective, i.e. arrival and leaving rates are balanced. In this case it is a crude way of modelling a system with a contention percentage of 100% under Amdahl's law, in that throughput is one over latency.

However this is an inaccurate way to model a system with locks. Amdahl's law does not account for coherence costs. For example, if you wrote a microbenchmark with a single thread to measure the lock cost then it is much lower than in a multi-threaded environment where cache coherence, other OS costs such as scheduling, and lock implementations need to be considered.

When modelling locks it is necessary to consider how contention and coherence costs vary given how they can be implemented. Consider in Java how we have biased locking, thin locks, fat locks, inflation, and revoking biases which can cause safe points that bring all threads in the JVM to a stop with a significant coherence component.